This invention relates generally to electronic warfare counter measures and more particularly to the generation of numerous coherent sinusoidal signals.
Electronic signals play an important role in modern combat situations. For example, radar is used for surveillance, targeting and guidance. Consequently, circumventing an adversary's radar is also important. One prevalent technique for circumventing radar is to build a decoy which generates signals to "fool" the radar.
One simple technique for fooling radar is to receive the signal transmitted by the radar and then retransmit the signal back at the radar. The radar might be fooled into thinking the retransmitted signal is actually a radar reflection from an airplane or a tank or some other important piece of military hardware (hereinafter referred to generically as a "target"). The drawback of this technique, however, is that sophisticated radars can distinguish between signals retransmitted by decoys and radar reflections from targets.
Radar reflections from a target are actually a super-position of reflections from various points on the targets. The points will be at different ranges from the radar such that the radar reflection is actually a superposition of reflections delayed in time relative to each other. Moreover, the reflection from each point will undergo a Doppler phase shift and each point could have a different Doppler shift. For example, the radar reflections from the exhaust plume of an airplane will have a different Doppler shift than the reflections from the body of the airplane.
In short, to fool a more sophisticated radar, a decoy must retransmit a superposition of signals, each one delayed and modulated with different frequencies. A drawback of such an approach is the number of oscillators needed to generate the retransmitted signal. A separate oscillator could be required to modulate each signal in the superposition. Constructing a decoy using that many oscillators is costly, technically complicated, and otherwise undesirable.
An alternative approach to using numerous analog oscillators is to store digitized samples of signals of the desired frequencies in memories. The samples are read out of each memory sequentially at constant time intervals and used in place of oscillator outputs. Such an approach, however, is still relatively complicated because of the need for memory for each frequency to be generated.